GPG-Based Password Wallet

Let's dig in to the script to see how it works. The first thing it does is use the dot operator to source a file called functions, which appears as shown in Listing 2. Having wallet source an external file (with the dot operator) is essentially equivalent to inserting the contents of the sourced file (~/bin/functions) at line 3 of wallet. Doing it this way allows other scripts to use the same code (a code library for shell scripts).

is_installed() {

    PROGRAM=$1

    PATHNAME=$( type $PROGRAM 2> /dev/null )
    if [ -z "$PATHNAME" ]; then
        echo "cannot locate $PROGRAM in path"
        exit 1
    fi

}

The functions file includes a function called is_installed, which uses the bash built-in type to see whether a program is installed. If is_installed doesn't find the program in your $PATH, is_installed prints an error message and calls exit, which terminates wallet. So, if you run wallet and it quits with an error like “cannot locate dialog in path”, you probably haven't installed the dialog package. Use your distribution's package management system (yum, apt-get, whatever) to install dialog and try again.

Lines 18 through 28 of the wallet script parse the command-line arguments using the getopts bash built-in. The while loop loops through the options specified by the string ec:. This means that wallet can accept the -e and -c options, and that the -c option requires an argument. As the while loop moves through the command-line arguments, the current option is assigned to the variable $OPTION, and any argument to the current option is assigned to the variable $OPTARG. Any unrecognized option results in an error message, and wallet exits. After the while loop completes, it's important to reset the $OPTIND variable (this is necessary after any getopts call).

Lines 37 through 45 of the wallet script verify that the encrypted file exists, and create the file if it doesn't exist already. The -f test checks to see whether $WALLET_FILENAME exists as a normal file. If not, the test fails, and wallet assumes you are running wallet for the first time and that wallet needs to set up the working environment. wallet uses the command substitution syntax for creating the directory in which the encrypted file should exist (line 40):

mkdir -p $( dirname $WALLET_FILENAME )

The command inside the $(...) runs first, and the result becomes the argument to mkdir. The dirname command returns the encrypted file's directory, and mkdir -p creates that directory (and any necessary parent directories).

Next, wallet needs to create the encrypted file (even though the unencrypted version will be empty). Line 41 uses mktemp to create an empty file in /tmp whose name ends in six randomly chosen characters. mktemp prints the name of the file it creates, so running this in a command substitution shell and assigning the result to $TEMPFILE puts the name of the temporary file in $TEMPFILE.

Now we see the first use of gpg. Line 42 uses gpg to encrypt the (empty) temporary file ($TEMPFILE) via symmetric encryption (gpg's -c option) and to write the encrypted file to $WALLET_FILENAME. wallet then deletes the temporary file. Because this is the first time wallet has run, it assumes that edit mode is appropriate and sets the $EDIT_PWFILE flag.

Line 52 uses the command substitution trick again, this time to prompt the user for the master password (used to encrypt the wallet file). The dialog man page describes the many ways that scripts using dialog can retrieve input from the user. This example uses dialog to create a simple password box. The --stdout option tells dialog to print the user's input (the master password) to standard output, so that it may be assigned to $PASSWORD.

Line 55 inspects the bash variable $?, which contains the exit code of the previous process (dialog, in this case). The convention is that an exit code of 0 indicates success (and wallet follows this convention in its own exit calls). If $? differs from 0 on line 55, this indicates that dialog encountered an error, and wallet terminates with an error message.

If $EDIT_PWFILE is 0 (line 65), then wallet is running in read-only mode:

echo $PASSWORD | gpg --decrypt --passphrase-fd 0 
 ↪$WALLET_FILENAME | less

This tells gpg to decrypt $WALLET_FILENAME and to read the password from standard input (fd 0). Piping $PASSWORD into gpg enables gpg to decrypt the wallet file without interactively asking the user for the master password. The output (the decrypted wallet file) is printed to standard output, which is piped into less, allowing the user to page through the passwords, run searches and so on. When the user closes less, wallet clears the screen and exits.

The rest of the script assumes that $EDIT_PWFILE is nonzero (that wallet is running in edit mode).